Surviving Baird circuitry in a first generation TV set.

[Catkins]

Hi,

Back in late 2014, Mark Parsons briefly appeared on the forum asking about his HMV 900. He later put the set up on ebay for sale, and I bought it.

Sadly it has mostly sat since that time waiting until I had time to start on a restoration. I have now started on the restoration.

Firstly, I'll provide the TRF and T/B numbers for Peter to update his list. Peter gives my brothers HMV 900 as owned by Rob (Draenog). In that vein can you update the owner as Phillip (Catkins). Thanks.

Unlike previous restorations, I'll try and post to this thread when I think I have something worthwhile to post.

Photo 1 is the TRF number, photo 2 is the T/B number.

TRF 1496
T/B 1044
Top side blanking.
Serial number H 6181

[Catkins]

Just a quick observation.

It would appear most sets were returned to EMI for renovation when television restarted in 1946. This set from various signs seems to have been renovated at that time by someone other than EMI. Included with the television was a battered copy of the "Television service sheet" from Sept 1938, and a parts list (reprint July 1946) overstamped with "The prices quoted for the spare parts listed herein no longer apply".

The renovations which can be dated to that time are a mixture of HMV parts, and other makes. The work is a bit clumsy, and so it is not of EMI origin.

Most intriguing is the replacement of the multi-capacitor block on the T/B chassis. As far as I can tell, most sets renovated by EMI have that capacitor block replaced by a paxolin strip with separate capacitors mounted on it. I daresay the general assumption is that the multi-capacitor blocks were by then unavailable.

But, this set has a genuine EMI replacement capacitor block dated DEC/46 (see photo). Which I think is quite intriguing.

[peter_scott]

Hi Phillip,

Thanks for your chassis numbers. I'm away from home for a couple of weeks but will update the list when I get back.

You might find that there is also a serial number on your power supply. Please take a look at the photo on the list for the possible location.

Regards,

Peter

[peter_scott]

Originally Posted by Catkins

But, this set has a genuine EMI replacement capacitor block dated DEC/46 (see photo). Which I think is quite intriguing.

Mine has a rather similar capacitor block dated Oct/46.

Peter

[Catkins]

Originally Posted by peter_scott

You might find that there is also a serial number on your power supply. Please take a look at the photo on the list for the possible location.

Hi Peter,

Sorry I was looking at an older version of your list (posted to the forum) which didn't have the "Power Sup" column.

The number is 000820.

Thanks

Phillip

[stevehertz]

Originally Posted by Catkins

Unlike previous restorations, I'll try and post to this thread when I think I have something worthwhile to post.

Personally I am happy to see all kinds of updates on these kind of restorations, they don't have to necessarily be deemed "worthwhile" by the restorer, as worthiness is a subjective issue. One can always duck out of the post if they're bored by it.

But anyway, best of luck with the restoration, all details happily read and absorbed.

[Catkins]

Originally Posted by stevehertz

But anyway, best of luck with the restoration, all details happily read and absorbed.

Thanks Steve,

I'm glad to hear that people find the posts interesting

[Andrewausfa]

Like Steve, I'm always interested. I don't do TVs personally but always like reading about the trials and tribulations of getting these sets to work again.

Andrew

[1100 man]

Hi Phillip,
I too have always enjoyed reading your posts, so please keep them coming!! I've never had the pleasure of working on any pre - war TV, so that makes threads about them even more interesting.

All the best
Nick

[Station X]

Originally Posted by stevehertz

Personally I am happy to see all kinds of updates on these kind of restorations, they don't have to necessarily be deemed "worthwhile" by the restorer, as worthiness is a subjective issue. One can always duck out of the post if they're bored by it.

But anyway, best of luck with the restoration, all details happily read and absorbed.

Please keep the updates coming Catkins. I'm never going to restore a vintage TV, but I enjoy reading threads like this. Makes a pleasant change from dried up grease, duff cartridges and skimming idler wheels!

[mark pirate]

I'm never going to restore a vintage TV, but I enjoy reading threads like this. Makes a pleasant change from dried up grease, duff cartridges and skimming idler wheels!

I have restored many 1940/50's TV's, but never had the pleasure of working on a prewar set.
I too am looking forward to reading about your set's restoration.

Mark

[Catkins]

Thanks everyone for the feedback

Sorry, I came down with a bad cold on Tuesday, and I'm only just getting over it now. I hopefully should send some posts in a couple of days.

[Catkins]

This post concerns some investigation which although necessary, largely turned out not to have any surprises. But following on from the feedback, this may still be interesting.

A couple of years ago, there was an interesting thread on VRAT about the survival of the Baird circuitry in early first generation EMI televisions. See https://www.radios-tv.co.uk/communit...00-mirror-lid/.

This thread was interesting because even from a first glance, it looked like my HMV 900 had most of the Baird circuitry surviving. So at this point in time, I thought it was worth my while spending time to see how much had in fact survived. Obviously, if it proved most of the Baird circuitry did survive, it raises the possibility of genuinely restoring dual-operation at some time in the future (IMHO only sets which do/did have the circuitry can be restored, otherwise it is a fake).

Obviously there entailed a search to find what documentation there was about the Baird circuitry. The "omnibus edition of pre-war television models" service manual, which is actually a misnomer as it only covers first generation sets, gives obsolete circuit diagrams which include the Baird circuitry. It also has a section entitled "Modifications to early models".

It transpires for HMV 900 models there are eight "modifications to early models", which include (as one of the eight) the step by step modifications to remove the Baird circuitry. Now, obviously, if the set still had the Baird circuitry, I decided I ought to check the status of the other seven modifications which are listed as "essential that these modifications should be fitted to all early production models".

Why you may ask? Well it should hopefully provide a bit more knowledge about the history and status of the set. Was the modification present at manufacture? Was the modification added at some later date, when it was serviced? Is there any modifications not done? The results can not only perhaps give a more accurate production date, but also an indication if/when the set was ever serviced (on the priviso if the set was returned to EMI for servicing, by default they'd ensure all modifications listed were made).

My results were:

* five of the modifications appear to be untouched, and to have been made in production

* the Baird circuitry is mostly complete (this will be dealt with in the next posts).

* two modifications had non EMI components

Now of those two it is too easy to conclude they were post-production modifications. They could have been present at production, and then replaced afterwards with later non EMI components. It is almost impossible to tell.

But, we have another slightly later HMV 900 in our collection. I thought it worthwhile to make a comparison.

Photos 1 and 2 are a photo of one of the modifications in this HMV 900, with a TCC capacitor (before and after restuffing, and cleanup etc for clairity). Photo 3 is the same circuit in the later HMV 900, which seems to be untouched since production. Now it should be fairly easy to see that the style of work is almost the same. This tends to imply the modification was present at production, and later replaced.

The other modification concerns the replacement of a 2-watt resistor with a 4-watt resistor. Again, photo 4 shows the replaced Welwyn capacitor in this HMV 900. The later HMV 900 also has this resistor replaced (photo 5), but, this was clearly done at a much later stage, presumably when the set was renovated in the 90s when owned by the previous owner.

As such is impossible to tell whether this is a post-production modification. It could have been present at production (i.e. 4-watt resistor) and then replaced afterwards. The fact it has also been replaced in the later HMV 900 might imply this is a high failure rate component.

It is not completely clear, but, my opinion is all the modifications listed pre-date the production of this set, except for the Baird circuitry.

The one thing which the investigation does tend to confirm, is there doesn't seem to be any post-production EMI servicing done on the set. All the work seems to date from the 1946 servicing work which wasn't done by EMI.

[Catkins]

The previous post dealt with checking whether the "early modifications" bar the Baird circuitry were present in the set. This post will deal with the Baird circuitry.

Photo 1 shows the obsolete ciruit diagram including the Baird circuitry. This consists of the Baird valve (circled in red) which supplied sync pulses that the Baird system lacked (no line sync pulses during frame sync pulse), plus four extra pieces of time-base circuit which were switched in/out by the "systems switch" (circled in green, yellow and orange), to accomodate the lower number of lines and frames.

Photo 2 shows the under-side of the Baird valve. What should be clear is it is completely intact. The heater/cathode/grid and anode connections are there, and the cathode bias resistor and bypass capacitor (circled in red) are also present.

Photo 3 shows two resistors soldered to the back of a potentiometer, these are the two resistors (R27 and R28) circled in green on the circuit diagram. You should also be able to see two wires joined to the resistors which disappear through a hole to the other side, these are the wires which originally connected to the standards switch.

Photo 4 shows the other side, showing the above two wires soldered to the chassis, earthing them and bypassing the resistors. This is what would have happened with the standards switch set to 405/50. These now permanently bypass the resistors making them unnecessary to remove.

The next post will deal with the circuits shown in yellow and orange on the circuit diagram.

[peter_scott]

Originally Posted by Catkins

there doesn't seem to be any post-production EMI servicing done on the set.

That is interesting and I would assume that the top mounted V8 cover plate was done before it left the factory and I guess the switch position is covered by a factory blanking plate like that pictured below.

Could you look closely at the mounting holes for the switch bridge piece? If these show screw threads then your timebase had been complete with switch before they modified it for shipment. Sets like mine that were never fitted with the dual standard capability simply have plain drilled holes without threading as in the photo below.

Peter

[Catkins]

Originally Posted by peter_scott

Could you look closely at the mounting holes for the switch bridge piece? If these show screw threads then your timebase had been complete with switch before they modified it for shipment. Sets like mine that were never fitted with the dual standard capability simply have plain drilled holes without threading as in the photo below.

Hi Peter,

One of the holes is being used to solder the wires to the chassis (with a screw in it).

But, the other hole does show screw threads, see the attached photos from different angles.

Regards

Phillip

[Catkins]

Originally Posted by peter_scott

That is interesting and I would assume that the top mounted V8 cover plate was done before it left the factory and I guess the switch position is covered by a factory blanking plate like that pictured below.

That was going to be the subject of another post

Photo 1 shows the top mounted V8 cover plate. If you compare it with another valve socket, you should be able to see the riveting is identical. The switch position is covered by a blanking plate.

Given I can't see any post-production EMI servicing on the set, the most likely explanation is the set was fitted with the Baird circuitry at production, but it was removed prior to shipment.

That would probably give the production date as late January 1937, given, the last Baird transmission was on 11th February. Obviously the dual EMI/Baird transmissions were supposed to be for six months. So, dating does depend on the gap between production and shipment, and on how much warning (if any), that EMI were given that the Baird transmissions were to be terminated early.

Phillip

[peter_scott]

Hi Phillip,

I think your H6181 is a very useful set for production dating and the factory fitted V8 top surface blanking plate places it very neatly in the January/February 1937 window. To preserve its history I hope that you will retain the plate with its original riveting over the socket.

This of course doesn't prevent you from making the set dual standard. You could add a small separate chassis for V8 if you were keen to operate with Baird frame pulses but as you know it's not essential for running the 900 at 240 line 25 Hz non-interlaced by generating the frame pulses with inset line pulses.

Peter

[SteveCG]

I hope this is not too silly a question but why would many TV sets be returned to EMI for renovation after the war? After all, wireless sets of that era ran for many years without needing 'renovation' and for TV there was no change to the broadcast standard.

[Station X]

Presumably because sets deteriorate when left unused for six years.

Things wear out through use and deteriorate through lack of use.